1. Compression stroke. During compression, the piston moves from the bottom dead center to the top dead center. This stroke has two functions: one is to raise the temperature of the air to prepare for the self ignition of the fuel; the other is to create conditions for the expansion of the gas.

When the piston goes up and the intake valve is closed, the air in the cylinder will be compressed. As the volume becomes smaller, the pressure and temperature of the air will increase. The pressure and humidity at the end of compression are related to the compression degree of the air, that is, the compression ratio.

2. Intake stroke. Its task is to fill the cylinder with fresh air. When the intake stroke begins, the piston is at top dead center, and there is still some exhaust gas in the combustion chamber of the cylinder. When the crankshaft rotates the elbow, the connecting rod makes the piston move from top dead center to bottom dead center. At the same time, the transmission mechanism connected with the crankshaft makes the intake valve open.

With the downward movement of the piston, the volume above the piston in the cylinder increases gradually: the air pressure in the cylinder is lower than that in the intake pipe, so the outside air is constantly filled into the cylinder.

3. Combustion expansion stroke. At the beginning of this stroke, most of the fuel injected into the combustion chamber is burned. A lot of heat is released during combustion, so the pressure and temperature of the gas rise sharply. The piston moves downward under the action of high temperature and high pressure gas, and makes the crankshaft rotate through the connecting rod to do external work. So this stroke is also called work or working stroke.

4. Exhaust stroke. The function of the exhaust stroke is to exhaust the expanded exhaust gas, so as to fill with fresh air and prepare for the intake of the next cycle. When the working stroke piston moves to the bottom dead center, the exhaust valve opens, the piston moves from the bottom dead center to the top dead center driven by the crankshaft and connecting rod, and the exhaust gas is discharged out of the cylinder.